Octatrienes, cyclization

When butadiene is treated with PdCU the l-chloromethyl-7r-allylpalladium complex 336 (X = Cl) is formed by the chloropalladation. In the presence of nucleophiles, the substituted 7r-methallylpalladium complex 336 (X = nucleophile) is formed(296-299]. In this way, the nucleophile can be introduced at the terminal carbon of conjugated diene systems. For example, a methoxy group is introduced at the terminal carbon of 3,7-dimethyl-I,3,6-octatriene to give 337 as expected, whereas myrcene (338) is converted into the tr-allyl complex 339 after the cyclization[288]. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

Problem 30.2 Draw the products you would expect from conrotatory and disrotatory cyclizations of (2Z,4Z.6/)-2,4,6-octatriene. Which of the two paths would you expect the thermal reaction to follow ... 

If a stepwise cyclization mechanism is assumed—for example, for /j-Cg—two octatriene intermediates may be formed, viz. 1,3,5-octatriene would lead to ethylbenzene, and 2,4,6-octatriene to o-xylene (Scheme II). The dehydrogenation of the latter would give octatetraene, which, in turn, gives styrene via vinylcyclohexadiene. Dehydrogenation and cyclization of octatriene were reported to compete over chromia and molybdena catalysts (67) with less hydrogen present (e.g., in a pulse system with in helium carrier gas), styrene formation is enhanced. 

The highly substituted 3,4-diazabicyclo[4.2.0]octatrienes (177) have been obtained by cyclization of cis- 3,4-dibenzoylcyclobutenes with hydrazine (69CB1928). The triene (R = H) undergoes thermal ring opening to give a vinylpyridazine, but with bridgehead substituents (e.g. R = Me) conversion to the substituted benzene (181) occurs in pyridine at 70-80 °C... 

Electrolytic reduction of 4-cyclooctenone leads smoothly in 64 % yield to cis-bicyclo[3.3.0]octan-l-ol.106) The thermolysis of octachlorocyclooctatetraene produces perchlorinated bicyclo[3.3.0]octatrienes,107,108) several of which have been examined in detail by X-ray structural methods.109) The 1,3,5,7-tetramethyl-cyclooctatetraene dication (57), generated by oxidation of the neutral hydrocarbon with antimony pentafluoride in S02C1F at —78 °C and below, is subject to dis-rotatory cyclization above — 60°.1 °) The latter step does not follow orbital sym-... 

Active Figure 30.5 Thermal cyclizations of 2,4,6-octatrienes occur by disrotatory ring closures. Sign in atwww.thomsonedu.com... 

The ergosterol system mentioned earlier falls in this category. Before the system is discussed, it is worthwhile to point out that, in general, substitution of even a simple group such as an alkyl seems to change the details of the photochemical primary processes in a startling manner among the dienes and trienes. Thus, in solution, 1,3,5-hexatriene cannot be converted to 1,3-cyclohexadiene photochemically, but 2,4,6-octatriene does cyclize photochemically to (rans-5,6-dimethyl-l,3 Cyclohexadiene under the same conditions. ... 

Example 6.3. The thermal cyclization of cyclooctatetraene to bicyclo[4.2.0l-octatriene. 

Probl m 30.4 What product would you expect to obtain from the photochemical cyclization of (2iS,4Z,6 )-octatriene Of (2E,4Z,6Z)-octatriene ... 

Thennal cyclizations of 2,4,6-octatrienes occur by disrotatory ring closures. 

Figure 10.27 summarizes the energy relationships for the four-, six-, and eight-electron systems relative to the polyenes. We see that for cyclobutene-1,3-butadiene, ring opening is favored and the is 32 kcal/mol. The is similar for the 1,3,5-triene cyclization (30 kcal/mol), but ring closure is favored. The E drops to 17.0 kcal/mol for the Z,Z-l,3,5,7-octatriene to 1,3,5-cyclooctatriene cyclization, whereas the E for the reverse reaction is 28.2 kcal/mol. 

The cascade is initiated by a conrotatory 8w-electron ring closure of the polyene carboxylic acids 20, 21 to the isomeric cyclo-octatrienes 22 and 23, respectively, which subsequently undergo a disrotatory 6 r-elec-tron cyclization to 24 and 25, respectively. Termination of the cascade by an intramolecular Diels-Alder reaction yields either the tetracyclic endiandric acid 26a, b or the bridged derivative 27. 

Interesting behaviour, dependent on the wavelength used, has been demonstrated for the bis diazo compound 492. Conventional irradiation at 254 nm leads to consecutive loss of nitrogen and cyclization to yield the mixture of isomers 493300,301 Using pulsed-laser irradiation at 248 nm affords the bis carbene 494, which dimerizes to yield the cyclo-octatriene 495. 

It is well known that Ni(0) catalyzes the cyclodimerization and cyclotrimer-ization of butadiene to form COD or CDT. On the other hand no cyclization of butadiene occurs with Pd(0) catalyst. Pd(0)-catalyzed dimerization of butadiene to form 1,3,7-octatriene (1) is the main reaction. 

Evidence for disrotatory cyclization in a simple case was provided by Marvell and by Vogel who found that trans, cis, ra/i5 -2,4,6-octatriene gives at least 99.9% pure c/5 -5,6-dimethyl-1,3-cyclohexadiene with an activation energy of 29.4 kcal/ mol and an entropy of activation of — 7 e.u. (Scheme 7.40). 

Bergamini et al. reported an unusual variation on the linear dimerization, wherein Pd-catalyzed dimerization in acetonitrile/water and in the presence of CO2 affords 37 selectively (52% yield, turnover number (TON) = 420). Under these conditions, only small amounts of the water-trapped product and 1,3,7-octatriene are observed as side products (Scheme 11). The formal intramolecular [3 + 2] cycloaddition product 37, particularly the 1,4-diene subunit imbedded in it, is reminiscent of the products obtained from cyclization of Tp-allylpalla-dium intermediates onto pendant alkenes, a process termed a palladium-ene reaction by Oppolzer and Gavdin, which has also been investigated by Negishi et al. and Trost and Luengo. One can account for the formation of 37 from the chelated Tr-allylpalladium intermediate 39 by ligand insertion to 40 followed by /3-hydride elimination. 

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